1. Field of the Invention
This invention relates to an engine cooling device, and more particularly to an engine cooling device of the kind including at least two engine cooling systems arranged around combustion chambers of an engine.
2. Description of the Prior Art
When the temperature of combustion chambers of an engine is excessively high, knocking occurs most frequently, and the intake air charging efficiency is lowered, giving rise to a reduced output. Further, since the operating valve system in the cylinder head generates frictional heat, it is generally desirable to cool an upper part of the combustion chambers at a relatively low temperature. On the other hand, it is generally desirable to cool a middle part and a lower part of the combustion chambers at a relatively high temperature. This is because, although frictional heat is generated at the sliding contact surfaces between the pistons and the inner walls of the combustion chambers, between the crank bearings and the crankshaft, etc. due to their state of fitting and oil film formation, it is considered rather expedient to maintain these parts at a relatively high temperature to reduce the frictional resistance thereby minimizing an undesirable reduction of the engine output.
An engine cooling device based on such an idea is known in which the cylinder head and the cylinder block of an engine are separately cooled by cooling water of a relatively low temperature and cooling water of a relatively high temperature respectively.
However, since cooling water started to boil and generate bubbles at about 100.degree. C., there is a limit in the relatively high temperature of cooling water cooling the cylinder block of the engine.
A device which overcomes this limitation has been proposed. According to, for example, Japanese Patent Laying-Open No. 43118/1985, engine lubricating oil is led to the cylinder block of a water-cooled engine to cool the cylinder block by the oil.
In the disclosed device, pressurized lubricating oil delivered from an oil pump is fed into the jacket of the cylinder block. However, because the pressure of the lubricating oil is high than that of cooling water pumped out to cool the water-cooled engine, the jacket must have a high mechanical strength resulting in an increased weight of the engine.